docs: def _parse_field_arguments(arg, faces, edges, coords):
      '''See mesh_register.'''
      if not hasattr(arg, '__iter__'):
          raise RuntimeError('field argument must be a list-like collection of instructions')
      pot = [_parse_field_argument(instruct, faces, edges, coords) for instruct in arg]
      # make a new Potential sum unless the length is 1
      if len(pot) <= 1:
          return pot[0]
      else:
          sp = java_link().jvm.nben.mesh.registration.Fields.newSum()
          for field in pot: sp.addField(field)
          return sp